High frequency electronic instruments such as oscilloscopes and logic analyzers require controlled impedance conductors for carrying high frequency signals without distortion or degradation. Controlled impedance coaxial cables provide connection to external interconnects to which external circuits may be connected. Microstrip transmission lines on printed circuit boards (PCB) provide controlled impedance by placing signal lines in a layer overlaying a return line or layer.
Creating a junction between coaxial cables and a printed circuit board has provided a well known challenge. Even when the impedance of a cable and the microstrip line to which it is attached are well matched, the junction itself will tend to present a different transition impedance. Such impedance variations tend to generate reflection of pulses, which degrades performance, particularly at the increasingly higher frequencies required in many applications. Using time domain reflectometery analytic tools, the limitations of existing cable-to-board interconnects become apparent.
A typical interconnect adequate for lower frequency usage employs a block to which the cable is connected. The block has multiple pins for soldering to PCB through-holes, with the outer shielding conductor (return line) of the cable connected to the block, and the inner conductor (signal line) connected directly to a through hole in the board. The inherent spacing between the through hole for the signal line and the holes for the return line will generally have a different impedance than desired, rendering this type of interconnect unsuitable for critical applications.
For critical applications, various interconnects have provided adequate performance, but at very high cost, or with other disadvantages. Such interconnects may require substantial time and skill for proper installation, and may be complex parts with a high manufacturing cost. Some interconnects may be precise, but of delicate construction that allows the characteristics to vary undesirably in response to mechanical forces and stresses. Other interconnect designs may require custom "trimming" in which a skilled worker tunes the characteristics of each interconnect to the desired characteristics.